The present invention relates to a silver complex diffusion transfer process (DTR process) and an image receiving material used therefor.
In DTR process the silver salt is transferred imagewise from a silver halide emulsion layer to an image receiving layer through diffusion and is converted to a silver image, in many cases, in the presence of physical development nuclei. For this purpose, the silver halide emulsion layer exposed imagewise is brought about in contact with the image receiving material in the presence of a developing agent and a silver halide complexing agent to convert the undeveloped silver halide to a soluble silver complex salt. Silver halide in the exposed portion of the silver halide emulsion layer is developed (chemical development) to silver, which therefore can no longer be dissolved and diffused. Silver halide in the unexposed portion of the silver halide emulsion layer is converted to a soluble silver complex salt, which forms a silver image, usually, in the presence of physical development nuclei. In the case of direct positive silver halide emulsion, action of the exposed portion and that of unexposed portion are reversed.
The DTR process has many uses. Especially, in the case of DTR process for reproduction of documents and production of block copies, it is required that the silver image formed on the image receiving material has high contrast and high maximum density and is of pure black and no fog occurs in non-image portion and whiteness of the non-image portion is high. However, it is known that the transferred silver image formed by such DTR process is generally not black, but has brown or other unacceptable colors. In order to solve these defects of DTR process, color toning agents are used. Various compounds are known as the color toning agents and a typical example thereof is 1-phenyl-5-mercaptotetrazole. However, when color tone satisfactory to some extent is obtained by using known color toning agents, there may occur considerable reduction in density or reduction in transfer rate.
Moreover, as processes for preparing physical development nuclei suitable for such image receiving materials, there have been known a process which comprises reducing a corresponding metal ion in an aqueous solution of a water-soluble polymer such as gelatin, a gelatin derivative, polyvinyl alcohol, carboxymethyl cellulose or the like to produce a metallic colloid dispersion and a process which comprises mixing a metallic ion with a solution of a soluble sulfide, a selenide or a telluride to produce a colloid dispersion of a water-insoluble metal sulfide, a metallic selenide or a metallic telluride.
However, the inventors have found that physical development nuclei prepared by these processes vary in their activity depending on the kind of the water-soluble polymer used and physical development nuclei of high activity generally tend to give transfer silver images of considerably reddish color tone with metallic gloss (bronzing) on the surface and on the other hand, physical development nuclei which give transfer silver images of satisfactory color tone with no bronzing phenomenon tend to be low in activity.
Furthermore, use of physical development nuclei in a larger amount within the range of commonly used amount can provide higher maximum density, but there are problems that considerable metallic gloss (bronzing) occurs on the surface of transfer silver images and besides, color tone is also deteriorated.
On the other hand, from the aspect of layer construction, it has been known that bronzing can be inhibited by providing a layer containing no physical development nuclei on an image receiving layer containing physical development nuclei. That is, Japanese Patent Kokai No. 1-94344 discloses a method for forming a sharp image having no metallic gloss in a short processing time and a short drying time by using an image receiving material in which the total solid coating amount of an image receiving layer and a water-permeable uppermost layer is 2 g/m.sup.2 or less.
Metallic gloss of transfer silver image can be controlled by providing the uppermost layer since physical development nuclei are not present on the surface and hence physically developed silver is not concentrated on the surface. However, even if conventionally used uppermost layers can inhibit metallic gloss, color tone of the produced transfer silver image is inferior or transfer density is insufficient.
In this case, although maximum density is somehow satisfactory, deterioration of color tone cannot be inhibited. Furthermore, when the two layers are provided, there is the problem of reduction in texture of black in the transfer silver image portion (for example, sordid black, cloudy black or brownish black when obliquely viewed though satisfactory black is seen when viewed from right above).
Further, the high whiteness of non-image portion on the image receiving material is also an important quality. For obtaining high whiteness, fluorescent brightening agents are used, but use of fluorescent brightening agents affects the quality of transfer silver image as shown in Japanese Patent Kokai No. 1-246539.
As explained above, it has been demanded to find methods of using fluorescent brightening agents which improve whiteness of non-image portion without affecting transfer silver image. Satisfactory methods have not yet been found.
Besides, rapid processing of photographic materials has recently been demanded and for this purpose, improvement of photographic materials per se to meet the rapid processing and improvement of processing solutions in conformity with such photographic materials have been attempted. For example, it is attempted to reduce amount of gelatin used in photographic materials and to make the layers thinner in order to attain rapid diffusion of processing solution. However, such methods cause not only deterioration of film properties, but also increase of fog. Therefore, film properties of high mechanical strength and causing no damage of photographic characteristics are demanded since now short-time processing at high temperatures using automatic processors or high energy processing solutions becomes popular.
There are quaternary pyridinium compounds described in Japanese Patent Kokai Nos. 56-36645 and 59-20369 as hardeners which do not cause various defects as mentioned above. It has become clear that these compounds not only have no aforementioned defects, but also extremely improve color tone and maximum density of image portion when they are used as hardeners in image receiving materials for silver complex diffusion transfer process.
In general, hardeners are required to provide coating solutions with time stability in use. Time stability of from several hours to several ten hours may sometimes be needed depending on manner of coating. This problem is solved, for example, by adjusting pH to 6 or more in case of generally used 2-hydroxy-4,6-dichloro-1,3,5-triazine.
However, it has been difficult to improve time stability by these methods using the quaternary pyridinium compounds described in the above-mentioned Japanese Patent Kokai Nos. 56-36645 and 59-20369.